Temperature compensated dielectric resonators

ABSTRACT

A dielectric resonator having a high effective dielectric constant, a high effective dielectric Q and a low effective temperature coefficient of dielectric constant is realized by a composite resonator comprising a first member including a paraelectric material such as, for example, TiO2 and a second member comprising either the ferroelectric material LiTaO3 or the ferroelectric material LiNbO3. Additionally resistive means applied to the ferroelectric material is used to reduce increased variations occurring in the effective temperature coefficient of the resonator during rapid temperature changes.

United States atent Plonrde [4 1 Nov. 11,1975

1 41 TEMPERATURE COMPENSATED DIELECTRIC RESONATORS [75] Inventor: James Kevin Plourde, Allentown,

[73] Assignee: Bell Telephone Laboratories,

Incorporated, Murray Hill, NJ.

221 Filed: Dec.22, 1972 2 11 Appl. No.: 317,385

[52] US. Cl. 333/82 BT; 333/83 T [51] Int. Cl. HOIP l/30;HO1P 7/06 [38] Field of Search 333/83 T, 82 ET [561 References Cited UNITED STATES PATENTS 3/1974 Konishi et al, 333/83 T OTHER PUBLICATIONS Boyd, C.R., Microwave Filters Utilizing Dielectric Resonators First Quarterly Report, Contract DA 28-043 AMCO2088(E) Rantec Corp., 84966, pp. 8 -9.

Smith et al, Temperature Dependence of the Elastic, Piezoelectric & Dielectric Constants of Lithium Tantalate & Lithiuim Niobate, J. R. of Applied Physics, Vol. 42, 5-1971, pp. 22l9--2230.

RESONANT FREQUENCY (MHZ 1 l l l TEMPERATURE (RC) Harrison, W. H, Microwave Filters Utilizing Dielectric Resonators, Final Report Contract DA28-043 AMC-O2088E Rantec Corp., 7-1967, pp. 5-10. Ohmachi et al, Dielectric Properties of LiNbO Single Crystal Up to 90C, Japan J of Appl. Physics 6, 1967 pp. 1467-1468.

Primary E.rantinerAlfred E. Smith Assistant Examiner-Wm. H. Punter Attorney, Agent, 0rFirmWilford L. Wisner [57] ABSTRACT 2 Claims, 9 Drawing Figures EFFECTIVE DIELECTRIC CONSTANT 4103f FOR LiTu0 -TiO RES0NATOR=49.5 EFFECTIVE DIELECTRIC Q FOR LiTClO mo RESONATOR=428O 3 2 4m 4'00 um 0 mo RESONATOR CONVENTIONAL COPPER CAVITY RESONATOR US. Patent N'v.11, 1975 Sheet2of5 3,919,672

EWQWTHWERROELECTRIC) EFFECTIVE DIELECTRIC CONSTANT FOR Li TO 0 TIO RESONATOR= 53 EFFECTIVE DIELECTRIC Q FOR .LiTGO "TiO RESONATOR=486O Ll TO O -TiO RESONATOR RESONANT FREQUENCWMHZ) t8 I I I 0 IO 4O TEMPERATURE (C) US. Patent N0v.11,1975 Sheet40f5 3,919,672

FIG]

G N VG MN T. RC E 0 TN J A E CH O B R l6 RWU 2 0 m D 5 TEN .WE 3 3 ONO IM G G Jan LRL O '3 O 2 m O l 9 8 m O W w W M 4 4 4 4 A ISZKVZMDOMEm .Z 2omwm TEMPERATURE (C) US. Patent Nov.11,1975 Sheet50f5 3,919,672

TEMPERATURE COMPENSATED DIELECTRIC RESONATORS BACKGROUND OF THE INVENTION This invention relates to microwave devices and, in particular, to temperature compensated dielectric microwave resonators.

In recent years, a great deal of effort has been expended in an-attempt to develop suitable dielectric resonators for applications at microwave frequencies. Such resonators can be dimensionally smaller than conventional microwave cavity resonators, and as a result, can be used to design low-cost microwave filters which would be compatible with present day microwave integrated circuitry. To date, however, microwave engineers and researchers have not been able to realize a dielectric resonator having microwave loss and resonant frequency stability characteristics which approach those of larger dimensioned conventional cavity resonators.

This failure to achieve a dielectric resonator having microwave characteristics equivalent to those of conventional resonators stems primarily from the factthat to achieve such characteristics in a dielectric resonator it is required that the material comprising the resonator exhibit certain dielectric properties at microwave frequencies, namely, a high dielectric constant, a high dielectric Q, and a low temperature coefficient of dielectric constant, the latter temperature coefficient being a measure of the rate of change of dielectric constant with respect to temperature. To satisfy the requirements of a high dielectric constant and a high Q, the resonator material should exhibit a dielectric constant greater than 40 and dielectric Q greater than 3000. Moreover, the requirement of a low temperature coefficient of dielectric constant is satisfied if the resonator material exhibits a temperature coefficient equal to or less than 34 parts per million per degree centigrade (ppm/degrees C).

Unfortunately, however, attempts at finding a dielectric material possessing the aforesaid three properties have turned out to be fruitless. Typically, materials have been uncovered with exhibit two of the required properties, those of high dielectric constant and Q, but not the third required property, that of a low temperature coefficient of dielectric constant. Such materials have been found to fall in the general class of materials known as paraelectrics, a typical material being TiO Having thus failed in their attempts to locate a suitable single dielectric material, microwave researchers looked to other avenues in search of a solution to the dielectric resonator problem. One particular avenue of research pursued by such researchers involved combining first and second members comprising, respectively, paraelectric materials having negative and positive temperature coefficients of dielectric constant in an attempt to realize a composite resonator having a low ef' fective temperature coefficient of dielectric constant and high effective dielectric constant and Q. It should be noted at this point that the terms effective dielectric constant, effective Q, and effective temperature coefficient of dielectric constant when applied herein to such a composite resonator refer to the dielectric properties that would be exhibited by a homogeneous dielectric resonator having equivalent dimensions and performance characteristics as those of the composite resonator.

lelectric constants of the former materials no suitable composite resonators have been realized' It is, therefore, a broad object of the present invention to provide a composite dielectric resonator having a high effective dielectric constant and Q and a low effective temperature coefficient of dielectric constant.

SUMMARY OF THE INVENTION In accordance with the principles of the present invention, the above and other objectives are realized by j a composite resonator comprising a first member, in-

\ eluding a paraelectric material having a high Qfand dielectric constant and a negative temperature coefficient of dielectric constant, and a second member including either the ferroelectric material LiNbO orthe ferroelectric material LiTaO More particularly, it has been recognized that the aforesaid two ferrelectrics, when appropriately oriented, exhibit high Qs, as well as high dielectricconstantsand positive temperature coefficients of dielectric constant at microwave frequen- 1 cies. Thus, by combining" a member comprising an appropriate volume ofeithfer one of these materials with a member including a para electric material having properties as above described, a composite structure having a high effective Q, high effective dielectric constant and a low effective temperature coefficient of dielectric constant is found to result. J

It has been further recognized, moreover, that a composite resonator formed in accordance with the above principles exhibits an effective temperature coefficient which is lower for slower changes in temperature than for faster changes in temperature. This result is believed due to a dielectric hysteresis effect which manifests itself during rapid temperature variations. Thus, in accordance with another aspect of the present invention, a resistive discharge path appropriately applied to the ferroelectric member of the composite resonator results in substantially eliminating the aforesaid hysteresis and thus in reducing the temperature coefficient of dielectric constant during rapid temperature changes.

BRIEF DESCRIPTION OF THE DRAWINGS A clearer understanding of the above-mentioned features of the present invention can be obtained by reference to the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 shows a first embodiment of a composite resonator in accordance with the principles of the present invention;

FIG. 2 illustrates the resonant frequency versus temperature characteristic of the resonator of FIG. 1;

FIG. 3 shows a second embodiment of a composite resonator in accordance with the principles of the present invention;

FIG. 4 is illustrative of the resonant frequency versus temperature characteristic of the resonator of FIG. 3;

FIG. 5 shows the resonant frequency hysteresis effect exhibited by the resonator of FIG. 1 during rapid temperature variations;

FIG. 6 illustrates a modification of the embodiment of FIG. 1 in which resistive paths are used to eliminate the resonant frequency hysteresis exhibited during rapid temperature variations;

FIG. 7 shows the resonant frequency versus temperature characteristic for the modified resonator of FIG.

FIG. 8 illustrates a modification of the resonator of FIG. 6;

FIG. 9 illustrates a modification of the embodiment of FIG. 3 in which resistive paths are used to eliminate the resonant frequency hysteresis exhibited during rapid temperature variation.

DETAILED DESCRIPTION In FIG. 1, a dielectric resonator 11, in accordance with the principles of the present invention, is illustrated. As shown, resonator 11 comprises two cylindrical members 12 and 13 which are joined along adjacent circular end surfaces 14 and 15, respectively. Cylindrical members 12 and 13 are of unequal longitudinal lengths L and L respectively. Moreover, each of the members is assumed to have a circular cross section of radius r, where r is greater than one half the sum of the lengths L and L The two cylindrical members thus form a composite cylindrical structure having a longitudinal axis, a radius r and a longitudinal length L equal to (L L In accordance with the invention, the cylindrical member 12 comprises a paraelectric material having a high dielectric constant, hihg dielectric Q and a negative temperature coefficient of dielectric constant. These parameters are designated in FIG. 1 as 6 Q and 1- respectively. A typical paraelectric material which can be used to form member 12 is TiO In further accord with the invention, the cylindrical member 13 comprises either the ferroelectric material LiNbO or the ferroelectric material LiTaO As is well known, each of these materials is anisotropic and thus each exhibits different dielectric constants and OS and different temperature coefficients of dielectric constant for waves having their electric fields polarized in different directions. It has been recognized, however, that each of these materials exhibits a particularly desirable combination of such parameters of waves at microwave frequencies having their electric fields in directions orthogonal to the optical axis of the material. In particular, it is found that for such waves a high dielectric constant and Q, in addition to a positive temperature coefficient of dielectric constant, are exhibited.

Advantageously, therefore, cylindrical member 13 has its optical axis parallel to its longitudinal dimension and thus in the direction of the resonator axis. The dielectric constant, dielectric Q and temperature coefficient of this member which are designated in the Figure in accordance with conventional crystallographic convention as 6 Q and 1' respectively, will thus have values as indicated above for all modes propagating along the resonator axis having their electric fields normal thereto, i.e., for all TE modes.

With members 12 and 13 thus chosen and arranged in the above-described manner, resonator 11 will have an effective dielectric constant, effective dielectric Q and effective temperature coefficient of dielectric constant for TE mode excitation which are functions primarily of the parameters s a O O T 1' and L and L More particularly, since TE modes propagating through the resonator will have their electric fields in the plane of the dielectric interface formed by surfaces 14 and 15 of members 12 and 13, respectively,

the composite resonator, to a first order approxima- Moreover, such analysis further shows that with the lengths L and L of resonator 11 selected in accordance with Eq. (I) and thus to realize a low temperature coefficient that the effective dielectric constant and Q of the resonator will be high. This result is due to the fact that the effective dielectric constant and Q, while also found to be functions of the lengths L and L have lower limits which are independent of the latter lengths and which are given respectively by the lesser of the two dielectric constants s and and the lesser of the two dielectric Qs, Q and Q Since, however, in accordance with the invention the materials comprising members 12 and 13 are such that both Q and Q11 are high and both s and e are high, the effective dielectric constant and Q of resonator 11 will also be high, even in the limiting case.

As indicated hereinabove, the realization of a composite dielectric resonator which exhibits a low effective temperature coefficient of dielectric constant and,

hence, changes with temperature of its effective dielec- I tric constant which are small results in achieving a resonator whose resonant frequency remains substantially constant with changes in temperature. This result can be readily understood by observing that the change in the resonator resonant frequency with temperature is primarily a function of the change with temperature of two resonator parameters, namely the resonator dimensions and the resonator effective dielectric constant. Since, however, the changes in the resonator dimensions resulting from a change in temperature are small in themselves, causing the changes in the effective dielectric constant of the resonator to be small will result in a resonant frequency whose changes with temperature are also small. The degree of compensation achieved by composite resonator 11 can thus be observed by examining a plot showing the changes in the resonator resonant frequency as a function of changes in temperature.

In FIG. 2 the resonant frequency versus temperature characteristic of resonator 11 is shown for a particular illustrative case in which members 12 and 13 are comprised, respectively, of the materials Ti0 and LiTaO and the resonator is excited in the lowest frequency TE mode, i.e., the T mode. The particular case shown is for a ratio of L /L 0.185, which ratio agrees quite favorably with the theoretical ratio of 0.l7 predicted by Eq. (I As can be seen, the resonant frequency characteristic of the composite LiTaO -TiO resonator shows a change over a temperature range from 0 to 70C which compares quite favorably with that exhibited by the resonant frequency characteristic (broken line curve in FIG. 2) of a conventional copper cavity resonator, thus indicating suitable compensation has been realized. Moreover, as also indicated, the aforesaid compensation has been achieved while additionally realizing a high effective dielectric constant equal to 49.5 and a high effective Q equal to 4280. Ad

fields normal to optical axis of members 32 and 33, when propagating in the latter members see the same high dielectric constant, high Q, and positive temperature coefficient of dielectric constant as seen by the TE modes propagating in member 13 of the resonator of FIG. 1. The aforesaid parameters have thus been similarly depicted in FIG. 2 by the symbols 6 Q and 7'.

With composite resonator 31 formed in the abovedescribed manner, TE modes propagating therethrough ditional parameters relating to the latter structure are 10 will have their electric fields in a directional normal to I should be noted, with respect to the embodiment of FIG. 1, that it was assumed that the radii of the cross sections of the two cylindrical numbers 12 and 13 were equal. The principles of the invention, however, are intended to apply, as well, to the case where the radii are not equal. Under such circumstances, the theoretical criteria for realizing a low temperature coefficient of dielectric constant is varied from that given by Eq. (I More specifically, instead of the ratioof the lengths L and L having to satisfy the relationship given by the right side of the Eq. (I the ratio of the volumes V and V of the two cylindrical members 12 and 13, respectively, must satisfy the right side of the equation. Thus, for this case Eq. (1) becomes In FIG. 3, a second composite dielectric resonator 31, in accordance with the principles of the present invention, is illustrated. Resonator 31 comprises two similarly shaped solid members 32 and 33. As shown, each of the latter members has a longitudinal length L and a cross section normal to the latter length which has the shape of a segment of a circle. The cross section of each member thus has a maximum vertical dimension t and a maximum transverse dimension w the latter two dimensions representing, respectively, the maximum thickness and maximum width of the member. The members 32 and 33 are arranged with their rectangular plane surfaces 32-1 and 33-1 separate but facing each other. Filling the region between the aforesaid plane surfaces is a third member 34 having a thickness t and a length and width equal to those of members 32 and 33. Resonator 31 thus formed has a maximum thickness equal to (2t ,+t,,) a maximum width equal to w and a length equal to L, the latter length dimension being assumed, in the present illustrative case, to be less than either of the aforesaid maximum thickness and width dimensions.

Member 34 of resonator 31 is comprised of the same material comprising member 12 in FIG. 1. Thus it has a high dielectric constant, high Q and a negative temperature coefficient of dielectric constant, these parameters being designated by 6 Q and T as in FIG. 1. Members 32 and 33, on the other hand, are comprised of the same material comprising member 13 in FIG. 1, i.e., of either the ferroelectric material LiTaO or LiNbO and are arranged with their optical axes parallel to the resonator axis, which is in the longitudinal direction. Thus, as in the previous embodiment, all the resonator TE modes, i.e., those having electric +289 ppm/degrees C the dielectric interfaces formed by member 34 and members 32 and 33. As a result, unlike resonator ll, resonator 31, to a first order approximation, appears electrically as a series combination of dielectric members. Analysis of resonator 31 as such an electrical combination, however, shows the resonator to have a high effective Q and a high effective dielectric constant for TE mode excitation, as was the case with resonator 11. Moreover, for such excitation the aforesaid analysis further shows that resonator 31 will additionally exhibit an effective temperature coefficient of dielectric constant which is substantially low when the ratio of the area A of the cross section of member 34 normal to its length to the total area A of the cross sections of members 32 and 33 normal to their lengths is related to the parameters s T 6,, and 1-,, in a specific manner. More particularly, the aforesaid relationship, to a first order approximation, is given by As with resonator 11, the degree of temperature compensation realized by the composite resonator 31 can be observed by noting the variation of the resonator resonant frequency with changes in temperature. FIG. 4 illustrates such variations for a typical case in which the resonator is excited in the TE mode and the cylindrical members 32 and 33 comprise LiTaO; and the member 34 comprises TiO As can be seen from the illustration, a total resonant frequency excursion of approximately 2 MHZ is found to occur over a temperature change of 60C. The latter frequency change agrees quite favorably with that occurring in the resonator of FIG. 1, and thus is indicative of the fact that suitable temperature compensation has been achieved. Moreover, as indicated in FIG. 4, a high effective dielectric constant and Q for the composite resonator have also been realized. For the particular situation illustrated, a ratio of It should be noted that the relationship defined by Eq. (3) was for the specific case of the member 34 having the same longitudinal dimension L as members 32 and 33. However, the principles of the invention apply as well when this is not the case. In such a situation, a compensated resonator can still be realized if the ratio of the volume V of member 34 to the total volume V of members 32 and 33 satisfies, to a first order approximation, the relationship 60TH li n (4) In developing the composite resonators of FIGS. 1 and 3, it was found that while the resonant frequency versus temperature characteristics corresponding to these resonators were adequately depicted by FIGS. 2 and 4, respectively, when the resonators encountered temperature changes over long time intervals and, thus, were in an equilibrium condition, such was not the case when the resonators were subjected to more rapid tem-' perature variations. More particularly, the characteristics of FIGS. 2 and 4 were obtained for slow temperature changes of the order of 3C/hr. For more rapid temperature changes, however, these characteristics become altered and exhibit a resonant frequency hysteresis effect which tends to increase the resonant frequency variation and thus decrease the frequency stability of the resonators.

Curve 51 in FIG. 5 illustrates the aforesaid resonant frequency hysteresis for the composite resonator whose resonant frequency characteristic for slow temperature variations is illustrated in FIG. 2. In particular, characteristic 51 is plotted for rapid temperature changes of the order of 45C/hr. As is readily apparent, the characteristic follows a typical hysteresis type loop, with the upper and lower portions of the loop corresponding to the resonant frequency variations during rapid increases and rapid decreases, respectively, in temperature. As is also apparent, the hysteresis type resonant frequency characteristic has the result of increasing the total frequency variation of the resonator over the depicted temperature range as compared to the variation experienced in the case of slow temperature variations. Thus, an increase in the total frequency excursion from 2 MHz to approximately 4 MHz is found to result.

As above-indicated, the aforesaid hysteresis effect and its resultant increase in resonant frequency variation will be exhibited by the resonators of FIGS. 1 and 3 only when the resonators are employed in situations where they will experience rapid temperature changes. While in most of these situations the increase in resonant frequency change might be tolerable, some situations may arise which dictate the desirability of a change which more closely approximates that occurring during slow temperature changes.

In such situations, it has been found that by modifying the resonators of FIGS. 1 and 3, the undesirable hysteresis effect can be substantially eliminated. More particularly, it has been recognized that the aforesaid result can be realized by connecting the end surfaces, i.e., those surfaces normal to the optical axis of each of the ferroelectric members of the composite resonator, through a resistive path having a resistivity and dimen sions such that electrical resistance of the path is less than the electrical resistance measured between the end surfaces of the ferroelectric member.

FIG. 6 shows the resonator of FIG. 1 modified to in clude such a resistive path. The basic resonator 61 in FIG. 6 is exactly the same as that of FIG. 1, and, hence, the same reference numerals have been used to indicate similar parts. The only additions to the resonator of FIG. 1 shown in FIG. 6 are the resistive paths 62, 63 and 64, each of which connects the two circular end surfaces 15 and 66 of ferroelectric member 13 and each of which has a resistance which is less than the resistance measured lengthwise across the latter member (i.e., the resistance measured between the two end surfaces 15 and 66 of the member). As shown, each of the resistive paths 62, 63 and 64 comprises a thin strip of material extending longitudinally along the length of the cylindrical surface of member 13 on opposing sides thereof and extending diagonally across each of the two circular end surfaces 15 and 66. Advantageously, therefore, the portions of each path lying along the aforesaid cylindrical surface will be normal to the electric fields of the TE mode propagating through the body of member 13. As a result, any coupling which might occur between this mode and the resistive paths is thereby minimized.

In FIG. 7, the resonant frequency versus temperature characteristic of the resonator of FIG. 6 is illustrated for rapid changes in temperature. The resonator dimensions and parameters for this illustrative case were the same as those pertaining to the illustrative case of FIG. 2. The material employed for the resistive paths 62, 63 and 64 was Ta N although other materials such as, for example, gold might also have been employed. As can be readily observed, the use of the resistive paths has substantially reduced the resonant frequency hysteresis, thereby resulting in a total resonant frequency excursion which closely approximates that occurring in FIG. 2 for slow temperature changes.

It should be pointed out that in the discussion of the embodiments of FIGS. 1 to 7 it was assumed that the ferroelectric members employed therein had first been polarized to establish similarly directed polarization fields in the members, the latter fields being parallel to optical axes of the members. Techniques for polarizing ferroelectric materials to achieve such a result are well known in the art. A typical arrangement is disclosed in Ferroelectric Domain Reversal in Lithium Metatantalate, A. A. Ballman and H. Brown Ferroelectrics Vol. 4, No. 3, pp. 189-195, November 1972.

As discussed above, the resonant frequency hysteresis occurring during rapid temperature changes can be substantially eliminated by applying a resistive path between the end surfaces of the ferroelectric members or members of the resonator. This result can be best understood by examining the various effects contributing to the change in the dielectric constant of the ferroelectric materials LiTaO and LiNbO during temperature changes.

More particularly, it has been recognized that when a temperature change is applied to either one of these materials, the dielectric constant of the material undergoes a change which is a function not only of the dielectric properties of the material, but also the pyroelectric properties thereof. Thus, it is found that when a temperature change occurs that the dielectric constant of the material, due to its dielectric properties, undergoes a direct change which is measured by the temperature coefficient of dielectric constant. However, it is also found that the pyroelectric properties of the material cause the dielectric constant to experience still a further indirect change whichalters the aforesaid temperature coefficient, thus causing the hysteresis type behavior discussed above.

Whether the aforesaid indirect change and resultant hysteresis behavior is, in fact, observable during any particular temperature change is, however, dependent upon how fast or slow the temperature change is occurring. This result can be explained by examining further the pyroelectric mechanism causing the indirect change. More particularly, as a result of the aforesaid mechanism, a temperature change applied to the Li- TAO (or LiNbO material causes positive and negative charge, respectively, to accumulate on opposing surfaces of the material which are normal to the polarization field, therein. Since, however, the material has a finite resistivity, the material itself provides a resistive discharge path between the aforesaid opposing surfaces and thus tends to dissipate the charge as it is being accumulated. When the temperature change is slowly applied, relative to the time constant of the aforesaid discharge path, charge is dissipated from the surfaces at a faster rate than it is accumulated and, as a result, no net accumulation of charge occurs. Thus, under such circumstances, no observable effect on the dielectric constant of the material results. However, when the temperature change is rapidly applied, relative to the aforesaid time constant, charge is dissipated from the surfaces at a slower rate than it is accumulated and, in this case, a net accumulation of charge on the surfaces does result. The aforesaid net charge, in turn, sets up a static electric field in the material along the direction of polarization and it is this field that provides the indirect change in the dielectric constant which results in the observed hysteresis behavior.

It is apparent from the above, therefore, that if during such rapid temperature changes the surfaces of the ferroelectric material where the charge is being accumulated are coupled by a resistive path whose resistance is less than that between the surfaces than a significant reduction in the inherent time constant governing the discharge of the charge will occur. As a result, the appearance of a significant net charge accumulation will be substantially prevented and the hysteresis behavior of the dielectric constant significantly reduced. Such is the effect realized by employing the resistance paths 62, 63 and 64 to couple the end surfaces 15 and 66 of the ferroelectric member 13 of the composite resonator illustrated in FIG. 6.

FIG. 8 shows the resonator of FIG. 6 modified such that the resistive paths need only be applied to the circular end surfaces of the ferroelectric members. More particularly, in FIG. 8, the ferroelectric cylindrical portion comprises two half cylindrical members 13-1 and 13-2 which have been poled parallel to the optical axis but in opposite senses relative to one another. This is illustrated in the Figure by the opposing polarization fields P and P respectively. A first set of resistive strips 62-1, 63-1 and 64-1, similar to those in FIG. 6, is used to connect the semicircular end surface 66-1 of member 13-1 to the adjacent semicircular end surface 66-2 of member 13-2. A second set of resistive strips 62-2, 63-2 and 64-2, similar to the first set of strips, 65

As a result of the manner in which members 13-1 and 13-2 are poled, the charge accumulated on the adjacentsurfaces 66-1 and 66-2 and the adjacent surfaces 15-1 and 15-2, which charge results in the abovediscussed dielectric hysteresis, will be opposite in sign. Thus, the resistive paths 62-1, 63-1 and 64-1 provide a low time-constant discharge path for the opposing charge accumulated on the two surfaces 66-] and 66-2,

and the paths 62-2, 63-2 and 64-2 for the opposing charge on the surfaces 15-1 and 15-2. The net result is thus similar to that achieved by the resistive strips in FIG. 6, and, as a result, a significant reduction in the hysteresis is observed.

FIG. 9 illustrates the resonator of FIG. 3 modified to also include resistive means for reducing the hysteresis effects exhibited by the resonator. In particular, the additions to the resonator of FIG. 3, shown in FIG. 9, are the resistive paths 92-1 and 92-2 and 92-3 which connect the two semicircular end surfaces 93-1 and 93-2 of ferroelectric member 32, and the resistive paths 94-1 and 94-2 and 94-3, which connect the two semicircular end surfaces 95-1 and 95-2 of ferroelectric member 33.

By causing each of the aforesaid paths to have a resistance which is less than the resistance measured lengthwise across its respective member, each path acts to reduce the hysteresis effect exhibited by resonator 91 in a similar manner as the resistive paths employed in the resonators of FIGS. 6 and 8.

As indicated hereinabove, the basic composite resonator configurations of FIGS. 1 and 3 can be approximated electrically as parallel and series combinations, respectively, of their constituent dielectric members. The two configurations thus indicate that a composite resonator having appropriate dielectric properties (high dielectric constant and Q and low temperature coefficient of dielectric constant) for resonator operation can be realized for the two extreme situations of parallel and series mixing of the dielectric properties of component members comprised of specific dielectric materials. Other types of dielectric mixing resulting from different combinations of these dielectric members will thus tend to result in effective dielectric properties for the combined structures which are somewhere in between those realized for the aforesaid extreme cases. Thus, such other composite structures will also have high effective dielectric constants and Qs and low temperature coefficients of dielectric constant and, as a result, will also be suitable for dielectric resonator operation.

In all cases, it is understood that the above-described arrangements are merely illustrative of a number of the many possible specific embodiments which represent applications of the present invention. Numerous and varied other arrangements can be readily devised without departing from the spirit and scope of the invention. For example, while members 12 and 13 of resonator 11 have been illustratively depicted as cylindrical in shape, other shapes for such members might have also been employed. A typical alternative shape which might be used for these members is that of a rectangular parallelepiped.

What is claimed is:

1. A dielectric resonator comprising a first member having a high dielectric constant, a

high Q, and a negative rate of change of dielectric constant with temperature over said frequency v 11 range. said first member including a paraclectric material;

a second member having a high dielectric constant, a high Q, and a positive rate of change of dielectric constant with temperature over said frequency range, said second member comprising LiTaO said first and second members being arranged to form a composite resonator structure having a high effective dielectric constant, a high effective and a rate of change of said effective dielectric constant with temperature which is substantially low over said frequency range, said second member being polarized in a direction parallel to the optical axis thereof.

2. A dielectric resonator comprising a first member having a high dielectric constant, a high Q, and a negative rate of change of dielectric constant with' temperature over said frequency range, said first member including a paraelectric material;

a second member having a high dielectric constant, a high Q, and a positive rate of change of dielectric constant with temperature over said frequency range, said second member comprising LiTaO said first and second members being arranged to form a composite resonator structure having a high effective dielectric constant, a high effective Q and a rate of change of said effective dielectric constant with temperature which is substantially low over said frequency range, in which:

said LiTaO member comprises two solid portions, said portions being of equal length and having similar cross sections normal to their lengths having the shape of segments ofa circle, each of said portions being arranged with a plane rectangular surface parallel to its length facing but separate from a plane rectangular surface parallel the length of the other portion and said paraelectric member is disposed in the region between the rectangular surfaces of said two portions and has a thickness equal to the separation between said portions, and

in which said second member has its optical axis and is polarized parallel to the lengths of said portions. 

1. A DIELECTRIC RESONATOR COMPRISING A FIRST MEMBER HAVING A HIGHDIELECTRIC CONSTANT, A HIGH Q, AND A NEGATIVE RATE OF CHANGE OF DIELECTRIC CONSTANT WITH TEMPERATURE OVER SAID FREQUENCY RANGE, SAID FIRST MEMBER INCLUDING A PARAELECTRIC MATERIAL; A SECOND MEMBER HAVING A HIGH DIELECTRIC CONSTANT, A HIGH Q, AND A POSITIVE RATE OF CHANGE OF DIELECTRIC CONSTANT WITH TEMPERATURE OVER SAID FREQUENCY RANGE, SAID SECOND MEMBER COMPRISING LITAO3, SAID FIRST AND SECOND MEMBERS BEING ARRANGED TO FORM A COMPOSITE RESONATOR STRUCTURE HAVING A HIGH EFFECTIVE DIELECTRIC CONSTANT, A HIGH EFFECTIVE Q AND A RATE OF CHANGE OF SAID EFFECTIVE DIELECTRIC CONSTANT WITH TEMPERATURE WHICH IS SUBSTANTIALLY LOW OVER SAID FREQUENCY RANGE, SAID SECOND MEMBER BEING POLARIZED IN A DIRECTION PARALLEL TO THE OPTICAL AXIS THEREOF.
 2. A dielectric resonator comprising a first member having a high dielectric constant, a high Q, and a negative rate of change of dielectric constant with temperature over said frequency range, said first member including a paraelectric material; a second member having a high dielectric constant, a high Q, and a positive rate of change of dielectric constant with temperature over said frequency range, said second member comprising LiTaO3; said first and second members being arranged to form a composite resonator structure having a high effective dielectric constant, a high effective Q and a rate of change of said effective dielectric constant with temperature which is substantially low over said frequency range, in which: said LiTaO3 member comprises two solid portions, said portions being of equal length and having similar cross sections normal to their lengths having the shape of segments of a circle, each of said portions being arranged with a plane rectangular surface parallel to its length facing but separate from a plane rectangular surface parallel the length of the other portion and said paraelectric member is disposed in the region between the rectangular surfaces of said two portions and has a thickness equal to the separation between said portions, and in which said second member has its optical axis and is polarized parallel to the lengths of said portions. 